In recent years, with an increased speed of a network caused by use of an ADSL or an optical communication technology, communication of image encoded data and audio encoded data through the network, using packets, is performed. A bidirectional communication system and a conference system using so-called VoIP (Voice over IP) and TVoIP (TV over IP) have become rapidly widespread.
There are some kinds of encoding methods used for communication of such image encoded data and audio encoded data. As an encoding method of a moving picture image, for example, a lot of methods of transmitting encoded data resulting from encoding by highly efficient compression based on inter-frame prediction are used. In these encoding methods based on the inter-frame prediction, an encoded image is predicted using frames which are temporally located before and after. Then, by encoding prediction parameters and a prediction residual image obtained by prediction of the encoded image, an information amount of moving picture image data highly correlated in the time direction is reduced. Further, by performing compression encoding with high efficiency by conversion encoding or quantizing the prediction residual image data, communication through a small transmission bandwidth is allowed.
Among the third-generation cellular phones are cellular phone terminals that support provision of video phone services in the circuit switching (CS) system. Further, there is also provided a gateway that connects a cellular phone terminal that may use these video phone services and a TVoIP client connected to an IP network. Then, using these cellular phone terminals and gateways, real-time communication over a CS network (circuit switching network) and a PS network (packet switching network) has been already performed.
When real-time communication is performed between the networks having the different transmission conditions (in terms of bandwidth, data loss, and error characteristic) as described above, a relay apparatus is used for connecting (relaying) both of the networks. In this case, the relay apparatus includes a jitter buffer that absorbs jitter on the networks, or performs delay reduction processing of reducing a transmission delay when the jitter is absorbed.
As the relay apparatus that relays encoded data, Patent Document 1, for example, describes an encoded stream relay apparatus that inserts encoding information obtained from an encoded stream into the encoded stream.    Patent Document 1: JP Patent Kokai Publication No. JP-P-2006-13583A (in paragraphs 0029-0042, FIG. 6)    Non-patent Document 1: H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications”, RFC 3550, July 2003, Internet <URL:http://www.ietf.org/rfc/rfc3550.txt>    Non-patent Document 2: TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU, “ITU-T Recommendation H.223”, 03/1996